Things just look better with transparent backgrounds. I don't think you have to worry about the image size, PeakD or any other frontend will always resize them for you. I always use the direct links and I never have issues with the images.
This post for example has a lot of linked cards https://peakd.com/hive-173286/@agrante/all-god-powers-in-gods-unchained. In this case they are in a table, but resizing also works if you use divs (2 images side by side or image and text).
If you just want one image as you have in this post, then you can use a little resizing trick mentioned for example in this Medium article: https://medium.com/@chrisgarrett/how-to-richly-format-your-hive-posts-with-html-and-markdown-bc3b348b002
You may have to use centering tags as well. But you can play with the '200x200' part of the URL to get any size you need.
That's very helpful. I wish I had known about this feature much earlier. It makes everything much faster, as you mention, and the image looks better. Thank you so much.